This invention relates to a programmable controller of a scanning type represented by a relay ladder diagram programming type.
Before programmable controllers were developed, the use of mechanical and simple sequence controllers, known respectively as a drum type sequencer and a rotary cam type sequencer, had been widely used.
The rotary cam type sequencer is designed so that a few cams are placed one over another on a rotary shaft and microswitches are arranged in positions corresponding to each cam. The microswitches are turned on and off in order by the rotation of the shaft. The drum type sequencer is also operated on the same principle as the rotary cam sequencer. In the drum type sequencer, the rotary drum is formed at its periphery with a groove into which is embedded a metal projection corresponding to the cam.
The sequence controllers of this kind are characterized in that their construction is simple and the sequence can be changed readily. However, there are parts which are mechanically worn and in addition, precise setting is difficult to make. Moreover, these controllers can only be applied to sequence control which is extremely small in scale and simple. At present, the aforesaid sequence controllers are rarely used.
On the other hand, most programmable controllers serve large-scale, complicated and high-degree control functions. Generally speaking, the large-scale control effected by programmable controllers includes a series of partly small-scale and simple sequence controls. It is very convenient that such a small-scale and simple sequence control be taken as a single control unit and programming is provided by way of expression similar to the above-described drum type sequencer.
A few programming types of programmable controllers are known. The most general type is the relay ladder diagram type. This is the type in which the sequence control system is expressed in the form of a circuit diagram using a relay to provide programming. Prepared sequence circuit diagrams are programmed directly or indirectly in preselected languages.
Even the conventional programmable controller of this kind can effect the same control as that of the drum type sequencer. However, programming for that purpose is not simple. Generally, a shift register command or stepping switch command is used or counter command and data comprison command are combined to program the same sequence control as that handled by the drum type sequencer.
To realize the control function of the drum type sequencer, there is originally required an inching signal for controlling the rotation of the drum and position information of the cam provided on the periphery of the drum.
Notwithstanding, if the function of the drum type sequencer is intended to be realized by the conventional programmable controller, an extremely lengthy program has to be set up by using commands as mentioned above. Therefore, programming is cumbersome and tends to make errors.
That is, despite the fact that the final control function comprises a simple sequence control, there is no exclusive-use command and therefore, the lengthy program, including a complicated intermediate logic process has to be set up.
Also, since the program includes such an intermediate logic process as described, the content of the program is very difficult to read. Accordingly, even in the case where the content of sequence control is partly changed, correction of the program for that purpose is not simple and errors are liable to occur.
As described above, the charateristic of the drum type sequencer, wherein the sequence change may be accomplished readily has not been introduced into conventional programmable controllers.